feat(transaction): Idempotent callbacks (immediate runs)

[dranikpg]

This PR generalizes the machanism of running transaction callbacks during scheduling, removing the need for specialized ScheduleUniqueShard/RunQuickie. Instead, transactions can be run now during ScheduleInShard - called "immediate" runs - if the transaction is concluding and either only a single shard is active or the operation can be safely repeated if scheduling failed (idempotent commands, like MGET).

Updates transaction stats to mirror the new changes more closely.

[dranikpg]

I want to rebase it on #2455 because currently the code is hard to deal with

[dranikpg]

I removed ScheduleSingleHop 😈

[romange]

please add a extensive commit description for the change. It's pretty fundamental. You change a two year old tx design, so worth at good explanation. You can squash commits too